Chapter 12

Princible of Homeostasis


Hormones and regulation of blood glucose

Diabetes and its control

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Princible of Homeostasis

Maintance of a constant internal enviroment

By maintaing a relatively constant enviroment (of the tissue fluid) for their cells, organisms can limit the external changes these celss experiance thereby giving the organismss a degree of independance.

What is Homeostasis

Maintaing the volume, chemical make up and other factors of blood and tissue fluid within restricted limits.

There are continous fluctuations, but they occur at a set point. Homeostasis is the ability to to return to that set point thus maintaining equilibrium.

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Importance of Homeostasis

Enzymes and other protiens are sensitve to changes in pH and tempuature. Wate potential of blood and tissue fluid should be kept constant to ensure cells dont burst of shrink due to a net movement of water (osmosis)

Maintaining a constant blood glucose concentration ensures water potential of the blood remains the same.

Independance of the external enviroment- a wide geographical range and therefore a greate chance of finding food, shelter

Mammals: homeostasis allows them to tolerate a wide range of conditions.

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Control Mechanism

Receptor- Takes in sensory information

Control centre- Determines the set point nd regualtes body's tempuature

Effector- Carries out the body's response

Variable- A factor in the body that can be modified by the effector

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Thermoregulation- Mechanisms for Gaining/Losing He


Production of heat= metoblism of food during respiration

Gain heat from enviroment= by conduction, convection, radiation


Evaporation of water




Endotherms- derive most of heat from metabolic activities (birds and mammals)

Ectotherms- obtain large proportion of their heat from outside body/enviroment (Lizard)

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Regulation of body tempuature in endotherms

Most heat gained through internal metabolic activities.

  • temp range= 35-42oC
  • compromise between higher tempuature where enzymes work more rapidly and the amount of energy needed ti maintain that tempuature
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Regulation of body tempuature in ectotherms

Body temp fluctuates with the enviroment.

  • controlled with exposure to the sun. Shelter from sun.burrows at night/ obtains heat from ground and very little frim raditaion
  • can sometimes change colour to alter heat that is raditated
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Conserving and gaining heat in response to a cold

Long term adaptions

  • Small surface area to volume ratio- therefore mammals and burds in cold eviroments are relative large
  • Smaller extremities- (eg. ears) and thick fur, feathers
  • Fat reserves to insulate body

Rapid changes

  • Vasocontriction- reducing the diameter if arteries/arterioles
  • Shivering- involentary rapid movements and contractions that release heat energy from respiration
  • Raising hair- enables a thick layer of still air to build up which acts as a good insulator
  • Behavioural mechanisms- basking in the sun
  • Deacreased sweating
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Loss of heat in response to a warm enviroment

Long term adaptions

  • Large surface area to volume ratio- smaller animals are found in warmer climates
  • Large extremities
  • Light coloured fur to reflect heat

Rapid changes

  • Vasodilation- arterioles increase in diameter, more blood reaches cappillaries, more heat is therefore radiated away
  • Increase sweating-heat energy is required to evaporate sweat. Energy for this comes from the body. Therefore, removes heat energy to evaporate water
  • Flatten body hair- hair erector muscles relax. Hairs flatten, reduces the insulating layer of air, so more heat can be lost to the enviroment
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Control of Body Tempuature

Mechanisms to contol body tempuature are coordinated by the hypothalamus in the brain.

It has a thermoregulatory centre divided into two parts:

  • heat gain centre- activated by a fall in body tempuature
  • heat loss centre- activated by rise in body tempuature

The hypothamus measures the tempuature of blood passing through it and thermoreceptors in the skin also measre the tempuature (core tempuature in the blood is more important than the temperature stimulating skin themoreceptors)

Impulses sent to the hypothalamus are sent via the autonomic nervous system

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Homeostasis diagram


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Hormones and regulations of blood glucose

Hormones are produced by endocrine glands, which secrete the hormones into the blood.

The hormones are carried in the blood plasma to target celss on which they act. The target calles have complemtory receptors on the cell surface membrane. Hormones are effective in small quantities, yet have widespread and long-lasting effects.

Some hormones work via the second messenge model:

  • The hormone binds to recpetors on the cell surcae membrane, forming a hormone receptor complex
  • The hormone-receptor complex activates an enzyme inside the cell that produces a secondary messenge chemical
  • The second messenger acts withing the cell produces and a series of changes

Both glucagon and adrenaline work by the secondary messenger model

The group of hormone producing cells in the pancreas are known as the Islets of Langerhans

  • a cells are larger and produce glucagon
  • b cells are smaller and produce insulin
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Blood glucose and variations in its level

Blood glucose comes fom 3 main sources:

  • Directly from the diet- resulting in the breakdown of carbohydrates
  • From the breakdown of glycogen (glycogenolysis)- glycogen is stord in the liver and muscle cells
  • From gluconegenesis- eg. Protien/amino acids and glycerol
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Insulin and beta cells in pancreas

Beta cells in the pancreas can detect an increase in glucose concentration in theblood and therefore release insulin.

When bound to receptor on the plasma membrane of teh cells, insulin brings out:

  • A change in the teritary structure of the glucose transport protien channels, causing them to change shape so as to allow more glucose into the cell
  • Increase the number of carrier molecules in the cell surface membrane
  • Activating enzymes involved in converting glucose to fat/glycogen
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Glucagon and A cells

When alpha cells detect a decrease un blood glucose concentration, they release glucagon

Alpha cells incrase blood glucose concentration by:

  • Activating enzyme that converts glycogen to glucose
  • Increasing the conversion of amino acids/glycerol to glucose

Adrenalinecan inactivate enzymes that convert glucose to glycogen

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  • Type 1- insulin dependant. Due to an autoimmune response where the body attacks B cells. The result is that the sufferer can't produce insulin.
  • Type 2- insulin independant. Glycoprotein receptors on the cells lose their responsiveness to insulin


  • Type 1- insulin injections. Insulin can't be taken orally as stomach acid will break it down. Dose of insulin must match the amount of glycose in the blood to avoid hypoglycaemia leading to unconciousness
  • Type 2- controlled by regular intake of carbohydrates and matching this to exercise. Some drugs can be used to stimulate insulin production or slow down the rate of glucose absorbtion in the intestine.
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Symptoms of Diabetes

High blood glucose level

Glucose in urine

Increased thirst/hunger

Urinate excessively

Genital itching


Weight loss

Blurred vision

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